From the days of the F2A things moved on, stuck around in part for the start of the DS, moved to expansion packs for the DS and have now more or less ceased. The days of a cable (be it to the cart or to the serial port of the GBA) went away and the better teams usually went to linkers/cradles instead and then to computer managed or DS managed.
After the F2A the flash cart makers realised that NOR memory was not getting any bigger (1024Mbit was as big as any commercial flash cart maker ever went) and NAND memory (used in everything that usually gets called flash memory) was not fast enough. This would be the likes of the EZ2 and In some cases they can still be found but for the most part they are rare as you like.
To that end they made carts with RAM instead and copied games to it at runtime. Most still had a small NOR section though. This is the likes of the EZ3 and they are rare as anything today.
The DS came along and people stuck with GBA slot stuff but rather than NAND chips and fancy USB cables/linkers they went to CF and various flavours of SD card with simple PC management tools. This is the likes of the EZ4, most dropped a bit of functionality compared to the full on GBA carts but still ran GBA ROM images and did so well, unless you had a supercard or a cut down GBA pack (the GBA hardware was expensive and if people wanted DS stuff they did not need it- the EZ4 lite compact, the M3 Pro and the supercard rumble being the big three here).
After people went to DS slot stuff you still needed a GBA slot device to run GBA so the teams instead made expansion packs and had DS programs manage the packs, ultimately store the saves and hold all the relevant data. They can work on a GBA but we do not suggest it.
Today it has been some years since the last DS intended GBA slot stuff was made, even longer since the dedicated GBA stuff and even the expansion packs are getting harder to find. Supposedly the EZTeam were coaxed out of hibernation for another spin of the EZ4, the result of which justinwebb presumably just linked you to, but for months they have been hard to find and in some cases commanding a reasonably high price.
Theoretically they probably could have made them work in a limited capacity without a miniSD but they need one as that is what stores the games, saves and such when it is not running. Best to actually get a miniSD rather than a microSD to miniSD adapter -- I do not know if it is the slots used or the adapters themselves but they tend to cause no end of hassle for EZ4 users, if you have one sitting around then by all means try it as some get on great but if you can lay your hands on a miniSD then do so. Also it is SD rather than SDHC so you are limited to 2 gigabytes.
